The present invention relates to a mass flowmeter that measures mass flow rate based on a Coriolis force generated in proportion to the mass flow rate of a fluid flowing through at least one vibrating measuring pipe. More specifically, the present invention relates to a mass flowmeter that is lightweight, simple in structure and easy to use, and which significantly improves the reliability and precision in mass flow rate measurement.
The Coriolis mass flowmeter facilitates high precision measurement of a mass flow rate, since it directly measures the mass flow rate based on the Coriolis force generated in proportion to the mass flow rate of a fluid flowing through one or more of vibrating measuring pipes. Generally, this kind of mass flowmeter may be classified into two categories. One is a mass flowmeter which includes a curved measuring pipe, such as a U or S-shaped pipe, and another is a mass flowmeter which includes a straight measuring pipe. The mass flowmeter may include two measuring pipes, which are arranged in parallel such that the fluid in question is distributed proportionally or connected in tandem and resonated (cf. Japanese Examined Patent Application No. H06-46167).
The mass flowmeter which includes one measuring pipe is described in the foregoing related application, and FIG. 3 illustrates in cross-section a single-measuring-pipe-type of the mass flowmeter. This type of mass flowmeter includes a support mechanism comprised of support members 3a, 3b, and a cylindrical tubular beam 4 for supporting a measuring pipe 2 at both ends thereof; a vibrator 5 for vibrating the measuring pipe 2; two vibration sensors 6a, 6b, for detecting vibrations in the measuring pipe 2; an inlet pipe 7a for guiding a fluid in question to the measuring pipe 2; an outlet pipe 7b for guiding the fluid in question out from the measuring pipe 2; and a housing 8 for supporting these components. This mass flowmeter facilitates sensing highly stable vibrations with high S/N ratio, by setting the frequency of the Coriolis vibrations generated in the measuring pipe to be higher than the natural frequency of the support mechanism. The Coriolis vibration frequency is set, for example, to be at or about .sqroot.2 times or more and, preferably, twice the natural frequency of the support mechanism.
The disclosure of the parent application Ser. No. 08/613,033 is incorporated in the present application.
The effects of producing Coriolis vibrations at a frequency higher than the natural frequency of the support mechanism, are further enhanced by utilizing the phase difference between the upstream and downstream vibration sensors, and by employing speed or acceleration sensors as the vibration sensors.
The Coriolis vibration frequency of the measuring pipe may be set at a level higher than the natural frequency of the support mechanism, by lowering the resonance frequency of the support mechanism, or by utilizing the higher order modes of the natural vibration, e.g., the third order mode or the fifth order mode, of the measuring pipe, as the Coriolis vibrations produced in the measuring pipe.
However, when a higher order mode of vibration is used for measuring pipes with various diameters designed for many purposes, the natural frequency of the higher order mode of vibration for the measuring pipe, not interfered by unwanted vibration modes, could not always be set at a level high enough than the natural vibration frequency of the support mechanism.
For example, when the rigidity of the support mechanism is lowered by reducing its thickness, the characteristic values, such as natural vibration frequencies in the circumferential direction of the support mechanism, tend to be lower, and the fundamental natural vibrations with the lowered frequency in the circumferential direction of the support mechanism and the higher order mode, e.g., third order mode, of the natural bending vibrations of the measuring pipe, tend to interfere with each other. Additional mass provided to the support mechanism leads to a complex structure, which further adds to the cost of the mass flowmeter, and unrealistically heavy mass must be handled.
In view of the foregoing, it is an object of the invention to provide a mass flowmeter which has a low manufacturing cost in view of its simple design, is lightweight and easy to use, and which significantly improves the reliability and precision in mass flow rate measurement.
It is another object of the invention to provide a mass flowmeter which facilitates to set Coriolis vibration frequencies at a level higher than the natural vibration frequency of the support mechanism.